1. Field of the Disclosure
The disclosure relates to a turbocharger for an internal combustion engine. More particularly, the disclosure relates to a means for controlling choke in a turbocharger having a radial flow compressor.
2. Description of Related Art
A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting the horsepower of the engine without significantly increasing engine weight. Turbochargers permit the use of smaller engines that develop the same amount of horsepower as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which contributes to the highly desirable goal of a cleaner environment.
Turbochargers typically include a turbine housing connected to the exhaust manifold of the engine, a compressor housing connected to the intake manifold of the engine, and a center bearing housing coupling the turbine and compressor housings together. A turbine wheel in the turbine housing is rotatably driven by an inflow of exhaust gas supplied from the exhaust manifold. A shaft rotatably supported in the center bearing housing connects the turbine wheel to a compressor impeller in the compressor housing so that rotation of the turbine wheel causes rotation of the compressor impeller. The shaft connecting the turbine wheel and the compressor impeller defines an axis of rotation. As the compressor impeller rotates, it increases the air mass flow rate, airflow density and air pressure delivered to the cylinders of the engine via the intake manifold.
The turbine wheel of a turbocharger rotates very rapidly. The rotational speed of a turbine wheel is size dependent, and smaller turbine wheels can rotate faster than larger wheels. A turbocharger turbine wheel used in conjunction with an internal combustion engine may reach circumferential tip speeds of 530 meters per second. The rapid rotation of the turbine wheel is directly transmitted to the compressor wheel which likewise rotates extremely rapidly. Accordingly, the gas coming off the compressor wheel is moving at a high velocity.
Once the air is accelerated by the compressor wheel and prior to being exhausted at the compressor exit, the air proceeds through a diffuser and into a volute formed in the turbocharger housing. The passageway from the compressor wheel into the volute is extremely narrow. The high velocity air is not capable of flowing through the narrow passage. The diffuser slows down the high-velocity air, largely without losses, so that both pressure and temperature are able to rise. The diffuser accomplishes this increase by essentially forcing the air from the compressor wheel to pass through the narrow passageway.
The mass flow of air through the compressor of a turbocharger increases up to a certain point at which the turbocharger compressor becomes so inefficient that the turbocharger chokes, or is unable to move more air. One of the causes of compressor choke is that at high mass flow rates the angle of attack of the flow of air upon the volute tongue becomes quite large which generates vortices and the air flow detaches from the volute tongue. The detached flow at the volute tongue restricts air flow from the compressor and can diminish the efficiency of the turbocharger.